This invention relates to a peelable lid structure and, in particular, to a method of making a peelable lid structure for use in the closing of food cans, for example.
Easy open ends using a ring pull tab for removal of a centre panel by tearing a score, are well established for use in the closing of cans, in particular those cans used for the packaging of pet food, soup and other food products. However, as food products typically require processing to comply with food safety standards, this type of easy open end is relatively thick in comparison with the side wall of the can, in order to withstand the temperatures and pressures inherent in the process.
Alternative closures which comprise a foil-type peelable lid structure which is adhered to a metal ring which is, in turn, seamed to the edge of the can body, have also been proposed. The peelable lid structure typically comprises a laminate structure of polypropylene, aluminium and varnish. The polypropylene layer is used for bonding to the metal ring. A tab which extends laterally from a centre panel of the peelable lid structure can be folded over the centre panel during retorting, or during handling operations such as seaming.
The aluminium layer performs as a barrier material and is generally about 60 to 70 microns thick. The thickness of this aluminium layer is not only dictated by the barrier requirements but also by the need to provide a “deadfold” to retain its folded position and prevent curling during sterilisation, This curling occurs at retort temperatures, which are typically 120° C. to 132° C. for processing products such as fish, pet foods or soup, due to differential shrinkage in layers which form the structure. The thick layer of aluminium thus retains the tab folded flat over the centre panel in particular during processing and seaming.
Both of the above types of peelable lid structure are costly to manufacture, in particular due to the high costs of metal used.